1. Field of the Invention
The present invention relates to a battery pack and a balancing method of battery cells.
2. Description of the Related Art
Generally, portable electronic equipment, such as notebook computers and hand-held power tools, is equipped with a rechargeable battery pack as a power source. The battery pack is composed of a plurality of battery cells and a battery monitoring unit for controlling overcharge or overdischarge of individual battery cells and calculating battery capacities.
Individual battery cells of the battery pack exhibit capacity variations which may occur due to various reasons associated with the manufacturing process. For these reasons, the battery pack inevitably undergoes variations in charge/discharge voltages of the battery cells during charge/discharge cycles. As a consequence, the battery pack may be susceptible to overcharge of a certain battery cell during the charge process and overdischarge of a certain battery cell during the discharge process. The occurrences of overcharge or overdischarge of certain battery cells in the battery pack result in a decreased capacity of the battery pack as well as degradation and lifespan reduction of the battery pack.
To prevent such occurrences, the battery monitoring unit is generally provided with a balancing circuit to minimize a voltage difference between the plurality of battery cells in series connection. That is, the battery monitoring unit senses a voltage of each battery cell and performs forcible discharge of a battery cell having a relatively high voltage when a voltage difference between the battery cells is larger than a reference value, such that all the battery cells have the same voltage. Further, balancing of the battery cells is initiated near the end of charge so as to save power.
For these reasons, the balancing of the battery cells in a conventional art has been carried out near the end of charge and with reference only to a voltage difference between the battery cells, so it cannot be said that perfect and accurate balancing of the battery cells was achieved. That is, a conventional battery cell balancing method is not efficient in terms of inhibiting overdischarge of the battery cells. Further, since the battery pack used in hand-held power tools is not provided with an overdischarge control unit, there is a higher chance for one of its battery cells to be in a deep discharge state.
As discussed above, the battery pack mounted in a special-purpose device such as hand-held power tool is typically not equipped with an overdischarge control unit system. For example, in order to enhance a maximum power output of the hand-held power tools, hand-held power tool makers typically require no installation of the overdischarge control unit in the battery pack. As a result, there is a high probability that a certain battery cell out of plural battery cells is under deep discharge conditions. Further, such an overdischarge event may bring about loading of a reverse voltage to a certain battery cell out of a plurality of series-connected battery cells. Once a certain battery cell out of the plurality of series-connected battery cells is exposed to a reverse voltage state, the corresponding battery cell then undergoes, for example, the opening of a current interruptive device (CID). In other words, when a reverse voltage is applied to a cylindrical lithium ion battery, the battery is in a highly unstable state, which triggers the operation of safety devices such as CID, for the safety of users. The CID is irreversibly operated. Therefore, the battery cell is not normally operated again once the CID is operated. That is, the battery pack should be replaced with a new one.